Thin film solar cells typified by amorphous silicon solar cells and compound thin film solar cells allow for significant reduction in material costs and production costs as compared with conventional crystalline silicon solar cells. In recent years, therefore, research and development have been rapidly conducted on these thin film solar cells. Among these thin film solar cells, a CIGS solar cell which is a type of compound thin film solar cell produced by employing Group I, III and VI elements as constituents and including a CIGS film composed of an alloy of copper (Cu), indium (In), gallium (Ga) and selenium (Se) as a light absorbing layer is particularly attractive, because the CIGS solar cell is excellent in sunlight conversion efficiency (hereinafter referred to simply as “conversion efficiency”) and is produced without the use of silicon.
As shown in FIG. 8, the CIGS solar cell typically includes a substrate 81, and a rear electrode layer 82, the CIGS film 83, a buffer layer 84 and a transparent electrically-conductive film 85 provided in this order over the substrate 81.
An exemplary method for producing the CIGS film (light absorbing layer) 83 of the CIGS solar cell is a so-called three-step method which is capable of imparting the CIGS film with a higher conversion efficiency. In this method, three steps are performed after the rear electrode layer 82 is formed over a front surface of the substrate 81. In the first step, In, Ga and Se are vapor-deposited on a front surface of the rear electrode layer 82 to form an (In,Ga)2Se3 film. In the second step, the temperature of the substrate 81 is increased to 550° C., and Cu and Se are further vapor-deposited, whereby a Cu-rich CIGS film intermediate product is formed. At this stage, two phases, i.e., liquid phase Cu(2-x)Se and solid phase CIGS, coexist in the CIGS film intermediate product, whereby crystal grain size is rapidly increased in the presence of Cu(2-x)Se. It is known that Cu(2-x)Se has a lower resistance and, therefore, adversely influences solar cell characteristics. In the third step, therefore, In, Ga and Se are further vapor-deposited to reduce the proportion of Cu(2-x)Se. Thus, the CIGS film 83 has a composition slightly rich in Group III as a whole. The CIGS film 83 thus formed by the three-step method has greater crystal grain diameters and yet has a thin film crystal structure having a crystallographically higher quality (see, for example, PTL 1).
The CIGS film 83 formed in the aforementioned manner has a V-shaped Ga/(In+Ga) ratio profile (so-called double-graded structure) such that the Ga/(In+Ga) ratio is progressively reduced along its thickness toward a predetermined thickness position 83a (see FIG. 8) from a back surface of the CIGS film 83 (an interface between the CIGS film 83 and the rear electrode layer 82) and is progressively increased toward a front surface of the CIGS film 83 from the predetermined thickness position 83a as shown in FIG. 9. The CIGS solar cell (see FIG. 8) employing the CIGS film 83 as the light absorbing layer has a higher conversion efficiency. The Ga/(In+Ga) ratio is herein defined as follows:
(A) the ratio of a gallium (Ga) atomic number concentration to the sum of an indium (In) atomic number concentration and the gallium (Ga) atomic number concentration.